Digging machines known as draglines are widely used to remove earth "overburden" and thereby expose and mine a mineral, e.g., coal, which is beneath but near the earth's surface. Almost invariably, draglines are used at sites remote from any well-equipped service facility or storehouse of repair parts.
Smaller draglines are crawler mounted much like a military tank and capable of movement in the same way albiet at much slower speeds. However, as draglines increased in size, crawler mounting was found to be impractical and in the early 1900's, the "walking" dragline was developed. The walking dragline is so named because it takes short "steps" and uses a walk leg mechanism (which resembles a human leg) to do so. A difference is that in a walking dragline, both legs step simultaneously.
A dragline is equipped with an angularly-extending boom from which is suspended a "bucket" having an open mouth and digging teeth, both toward the main portion of the machine. Overburden is removed by placing the bucket on the ground at a point distant from the machine and pulling it toward the machine, filling the bucket in the process. Once filled, the machine pivots about a central axis and the bucket emptied, at a spoil pile somewhat away from the area being excavated.
In other types of digging machines, e.g., a power shovel or backhoe, the bucket is attached to a rigid arm and can be forced through the material being removed. A dragline bucket (and the manner of bucket attachment) differ from such machines in that the dragline bucket is attached to the machine solely by flexible cables, chains and the like. Therefore, the weight of the bucket and the design and arrangement of its teeth (along with other factors) are important in configuring a dragline bucket which digs efficiently. An example of a dragline bucket is shown in U.S. Pat. No. 4,791,738 (Briscoe) and in trade literature filed with this specification.
As the dragline bucket is pulled toward the machine, it is generally horizontal for substantially complete filling. To empty the bucket, the operator actuates controls which tips such bucket to a near-vertical position with the teeth pointing downward. One component of the cable, chain and linkage arrangement connecting the bucket and the machine and used to control bucket "attitude" or orientation is called a dump block.
A dump block is a pulley-like device which, unlike a block-and-tackle, provides no mechanical advantage but which is used to change the direction of the force exerted by a flexible cable. Examples of dump blocks are depicted in the aforementioned Briscoe patent and literature.
Known dump blocks are relatively heavy, complex and expensive to manufacture. For example, side plates and sheaves are often made from castings and compared to other ways to make such parts, metal casting is a relatively expensive process. And such castings need to be machined before they can be assembled. Such side plates often require that a pocket be machined in the plate to receive and hold one end of a pin held in place by a through-bolt.
Similarly, at least the hub of the sheave casting needs to be machined to fit over a sleeve-type bearing or to receive the cup of a roller bearing. The pin itself may also require machining to fit properly with seals and bushings.
While such dump blocks have been generally satisfactory for their intended purpose, they tend to be characterized by certain disadvantages. In one type of common dump block, there is well in excess of 30 separate parts. Since dump blocks are frequently serviced "on site," this multiplicity of parts (some of which are rather small) almost assures that a part will be dropped in the dirt or lost during field service. And, of course, such multiplicity of parts complicates the matter of keeping a proper inventory of repair parts.
Yet another disadvantage is that the bearings (of whatever type) are subject to relatively rapid wear due to contaminants--dirt, rock dust and the like--in the operating environment. In an effort to prolong the life of such bearings, the dump block is configured so that such bearings can be greased periodically. But even at that, disassembly and repair tends to be relatively frequently required.
An improved dump block which is made of a minimum number of easily-made and relatively-large parts, which maximizes parts "commonality," which does not need lubrication, which takes advantage of relatively new materials, which is relatively light in weight and which is easy to assemble and disassemble would be an important advance in the art.